Measurement increased how much of the activity people did and decreased how much they enjoyed it. The effect persisted after measurement stopped: people read less afterwards, and in one experiment avoided reading altogether.
How it was done
Six experiments in which people did an ordinarily enjoyable activity — walking, reading — with or without their output being measured.
What it means
Drawing attention to output turns an enjoyable activity into work. Once it is work, the intrinsic reason for doing it erodes, and the extrinsic one disappears the moment the counter does.
The lingering effect is the damaging part. This is not a neutral motivator with an upside while it runs — it leaves the activity in a worse state than it found it.
Substitute "friendship" for "reading" and read the finding again. That is the proposition behind every streak, contact score and relationship-health percentage in this category.